1. Field of the Invention
The invention relates to a method of driving a plasma display panel (PDP) which is one of flat display panels which can be readily formed in a larger size, and more particularly to such a method which makes it possible, after priming discharge or preliminary discharge has been generated in all of cells, but prior to addressing action carried out for determining a cell or cells which emit(s) light, to generate priming erasing discharge for controlling wall charges generated by the preliminary discharge.
2. Description of the Related Art
A plasma display panel is presently used in various fields such as a personal computer, a display of a work-station, or a television set hung over a wall. A plasma display panel can be structurally grouped into a direct current (DC) type panel in which electrodes are exposed to discharge gas, and an alternate current (AC) type panel in which electrodes are covered with a dielectric film, and hence, are not exposed to discharge gas. An alternate current (AC) type panel is grouped further into a memory operation type panel which makes use of a memory function caused by a charge-accumulation function of the dielectric film, and a refresh operation type panel which does not make use of the memory function.
FIG. 1 is a cross-sectional view of a conventional AC type plasma display panel.
The illustrated plasma display panel includes a front substrate 1 and a rear substrate 2 both of which are composed of glass.
On the front substrate 1 are formed a plurality of scanning electrodes 3 and a plurality of sustaining electrodes 4 all extending in a direction perpendicular to a plane of FIG. 1. Each of the scanning electrodes 3 is equally spaced away from each of the sustaining electrodes 4. A dielectric layer 5a is formed on the front substrate 1 such that the dielectric layer 5a entirely covers the scanning and sustaining electrodes 3 and 4 therewith. A protection layer 6 is formed entirely over the dielectric layer 5a. The protection layer 6 is composed of magnesium oxide (MgO), and protects the dielectric layer 5a from discharge generated in a discharge space 9 defined between the front and rear substrates 1 and 2.
On the rear substrate 2 is formed a plurality of data electrodes extending in a direction perpendicular to a direction in which the scanning and sustaining electrodes 3 and 4 extend. The data electrodes 8 are covered with a dielectric layer 5b. Phosphor 7 is coated over the dielectric layer 5b for converting ultra-violet light generated by discharge, into visible light. A color plasma display panel is fabricated, if red, green and blue phosphors are coated in every three cells.
Between the dielectric layers 5a and 5b is formed a partition wall 10 for defining discharge spaces 9 and partitioning cells. A discharge gas composed of He, Ne and Xe is introduced into each of the discharge spaces 9.
FIG. 2 is a plan view of the scanning, sustaining and data electrodes 3, 4 and 8 of the plasma display panel illustrated in FIG. 1.
As illustrated in FIG. 2, first to m-th scanning electrodes Si (i=1, 2, - - - , m) are formed to extend in a column direction, and first to n-th data electrodes Dj (j=1, 2, - - - , n) are formed to extend in a row direction. A cell is formed at each of intersections of the scanning and data electrodes. First to m-th sustaining electrodes are formed to extend in a column direction in parallel with the scanning electrodes Si. Each of the first to m-th scanning electrodes Si and each of the sustaining electrodes Ci make a pair.
FIG. 3 is a timing chart showing waveforms of driving voltages to be applied to the scanning, sustaining and data electrodes 3, 4 and 8. Hereinbelow is explained a method of driving the AC type plasma display panel, with reference to FIG. 3.
First, a first preliminary discharge pulse 11a having a sign which is negative with respect to a base voltage of a sustaining electrode is applied to the sustaining electrodes 4, and a second preliminary discharge pulse 11b having a sign which is positive with respect to a base voltage of a sustaining voltage is applied to the scanning electrodes 3. As a result, a voltage difference exceeding a threshold voltage at which discharge starts is applied across the scanning electrodes 3 and the sustaining electrodes 4, and thus, discharge is compulsorily generated in all cells.
The first preliminary discharge pulse 11a is rectangular in shape. Hence, a voltage drastically varies at leading and trailing edges of the first preliminary discharge pulse 11a. The second preliminary discharge pulse 11b is serrate in shape, and hence, a voltage gently varies at a leading edge of the second preliminary discharge pulse 11b. An inclination of the leading edge of the second preliminary discharge pulse 11b is set smaller than about 10 V/μs.
Then, a preliminary erasing discharge pulse 12 having a sign which is negative with respect to a base voltage of a scanning electrode is applied to the scanning electrodes 3 for generating discharge in all cells to thereby put wall charges into an initial state for generating writing discharge afterwards.
The preliminary erasing discharge pulse 12 is serrate in shape, and hence, a voltage gently varies at a leading edge of the preliminary erasing discharge pulse 12. An inclination of the leading edge of the preliminary erasing discharge pulse 12 is set smaller than about 10 V/μs.
Discharge generated by the first and second preliminary discharge pulses 11a and 11b is called preliminary discharge, and discharge generated by the preliminary erasing discharge pulse 12 is called preliminary erasing discharge. Subsequent writing discharge is stably generated by virtue of the preliminary discharge and preliminary erasing discharge.
After the preliminary discharge and preliminary erasing discharge have been generated, a scanning pulse 13 is applied to each of the scanning electrodes S1 to Sm at different timings from one another. The scanning pulse 13 has a sign which is negative with respect to a base voltage of a scanning electrode.
In synchronization with the scanning pulse 13, a data pulse 14 is applied to the data electrodes D1 to Dn in accordance with image data. The data pulse 14 has a sign which is positive with respect to a base voltage of a data electrode. An oblique line in each of the data pulses 14 indicates whether presence or absence of the data pulse 14 is determined in accordance with presence or absence of image data for a cell.
In a cell in which the data pulse 14 is applied to the data electrode 8 while the scanning pulse 13 is being applied to the scanning electrode 3, discharge is generated in the discharge space 9 defined between the scanning electrode 3 and the data electrode 8. In contrast, if the data pulse 14 is not applied to the data electrode 8 while the scanning pulse 13 is being applied to the scanning electrode 3, discharge is not generated in the discharge space 9. Image data is written into a cell in accordance with presence or absence of the discharge, the discharge is called a writing discharge.
In the above-mentioned writing discharge, the discharge generated between the scanning electrode 3 and the data electrode 8 triggers discharge between the scanning electrode 3 and the sustaining electrode 4. In order to cause the discharge between the scanning electrode 3 and the sustaining electrode 4 to be stably generated, a voltage difference between the scanning electrode 3 and the sustaining electrode 4 in the writing discharge may be increased by applying a bias voltage or scanning sub-pulse 17 to the sustaining electrodes 4. The scanning sub-pulse 17 has a sign which is positive with respect to a base voltage of a sustaining electrode. In order to shorten an amplitude of the scanning pulse 13, a bias voltage or scanning base pulse 18 may be applied to the scanning electrodes 3. The scanning base pulse 18 has a sign which is negative with respect to a base voltage of a scanning electrode.
In a cell in which the writing discharge has been generated, positive charges called “wall charges” are accumulated on the dielectric layer 5a above the scanning electrodes 3. In contrast, negative wall charges are accumulated on the dielectric layer 5b above the data electrodes 8. Thereafter, a positive voltage caused by the positive wall charges accumulated on the dielectric layer 5a above the scanning electrodes 3, and a first sustaining pulse 15a having a negative sign and applied to the sustaining electrodes 4 overlap each other with the result that first discharge is generated.
If discharge is generated also between the scanning electrode 3 and the sustaining electrode 4 during the wiring discharge, negative wall charges are accumulated on the dielectric layer 5a above the sustaining electrodes 4 by the writing discharge. As a result, a positive voltage caused by the positive wall charges accumulated on the dielectric layer 5a above the scanning electrodes 3 and a negative voltage caused by the negative wall charges caused by the dielectric layer 5a above the sustaining electrodes 4 are added to the first sustaining pulse 15a, resulting in that first discharge is generated.
After the first discharge has been generated, positive wall charges are accumulated on the dielectric layer 5a above the sustaining electrodes 4, and negative wall charges are accumulated on the dielectric layer 5a above the scanning electrodes 3. A second sustaining pulse 15b to be applied to the scanning electrodes 3 is added to a voltage difference between the above-mentioned positive and negative wall charges, resulting in that second discharge is generated.
In the same way as mentioned above, a voltage difference caused by positive and negative wall charges accumulated by n-th discharge is added to a (n+1)-th sustaining pulse 15b, resulting in that discharge is kept generated. Hence, discharge caused by the above-mentioned action is called sustaining discharge. A luminance is dependent on the number of sustaining discharges.
If the sustaining pulses 15a and 15b are designed to have a voltage at which discharge is not generated merely by applying the sustaining pulses 15a and 15b to the sustaining and scanning electrodes 4 and 3, first sustaining discharge is not generated even if the first sustaining pulse 15a is applied to the sustaining electrodes 4 in a cell in which the writing discharge has not been generated, because a voltage caused by wall charges is not generated before applying the first sustaining pulse 15a to the sustaining electrodes 4. Accordingly, subsequent sustaining discharges are not generated.
After the sustaining pulses 15a and 15b have been applied to the sustaining electrodes 4 and the scanning electrodes 3, respectively, a sustaining erasing pulse 16 having a sign which is negative with respect to a base voltage of a scanning electrode is applied to all of the scanning electrodes 3 to thereby generate discharge in a cell in which the sustaining discharge has been kept generated. As a result, a wall charge profile is initialized. The sustaining erasing pulse 16 is a serrate pulse having a leading edge varying smaller than about 10 V/μs. Discharge caused by the sustaining erasing pulse 16 is called sustaining discharge erasion.
With reference to FIG. 3, a period in which the preliminary discharge pulses 11a and 11b and the preliminary erasing discharge pulse 12 are applied to the sustaining electrodes 4 and the scanning electrodes 3 is called a preliminary discharge period, a period in which the scanning pulse 13, the data pulse 14, the scanning sub-pulse 17 (if necessary), and the scanning base pulse 18 (if necessary) are applied to the electrodes is called a scanning or addressing period, a period in which the sustaining pulses 15a and 15b are applied to the sustaining and scanning electrodes 4 and 3 is called a sustaining period, and a period in which the sustaining erasing pulse 16 is applied to the scanning electrodes 3 is called a sustaining erasing period. A combination of a preliminary discharge period, a scanning period, a sustaining period and a sustaining erasing period makes a sub-field.
A method of displaying images at a certain gray scale in a conventional plasma display panel is explained hereinbelow with reference to FIG. 4.
A field defined as a period in which one picture is to be displayed is divided into a plurality of sub-fields. For instance, a field is 1/60 seconds, and is divided into four sub-fields. Each of the sub-fields has such a structure as illustrated in FIG. 4, and is controlled to be turned on or off independently of other sub-fields. The sub-fields have sustaining periods different from one another. In other words, the sustaining pulses 15a and 15b are applied to the sustaining and scanning electrodes 4 and 3 in each of the sub-fields in the different numbers from one another, and hence, the sub-fields provide different luminances from one another.
When a field is divided into four sub-fields as illustrated in FIG. 4, it is assumed that a ratio in luminance obtained when light is emitted solely in each of the sub-fields is set 1:2:4:8, for instance. Thus, it would be possible to display images at sixteen (16) luminance ratios from zero (0) to fifteen (15) in accordance with a combination of four sub-fields in each of which light is emitted or not. Herein, if no light is emitted in all of the sub-fields, a luminance ratio would be zero, and if light is emitted in all of the sub-fields, a luminance ratio would be fifteen.
If a field is divided into N sub-fields, and a luminance ratio in the N sub-fields is set at 1 (=20): 2 (=21): - - - : 2(N−2): 2(N−1), it would be possible to display images at 2N gray scales.
However, the above-mentioned conventional method of driving a plasma display panel is accompanied with a problem that excessively intensive discharge might be generated, when the preliminary erasing discharge pulse 12 having a leading edge varying at a rate smaller than 10 V/μs is applied to the scanning electrodes 3, resulting in that sustaining discharge is generated regardless of whether the writing discharge has been generated, in a cell in which excessively intensive preliminary erasing discharge has been generated.
Japanese Patent Application Publication No. 2001-184023 has suggested a display unit including a plurality of first electrodes arranged in a first direction, a plurality of second electrodes arranged in a second direction perpendicular to the first direction, a plurality of third electrodes each of which makes a pair with each of the first electrodes, and a controller which adjusts a wall voltage difference between the first and third electrodes, and further adjusts a wall voltage difference between the first and second electrodes independently of the adjustment of the wall voltage difference between the first and third electrodes, before addressing discharge is generated between the first and second electrodes.
Japanese Patent Application Publication No. 2001-210238 has suggested an AC type plasma display panel including a first substrate, and a second substrate facing the first substrate with discharge space being sandwiched therebetween. On the first substrate are formed a first electrode, a second electrode extending in parallel with the first electrode, and a dielectric layer covering the first and second electrode therewith. On the second substrate is formed a third electrode extending in a direction perpendicular to a direction in which the first electrode extends. A distance between the first and second electrodes is set greater than a height of the discharge space.
Japanese Patent Application Publication No. 2001-242824 has suggested a method of driving a plasma display panel including a discharge cell which includes a first electrode and a second electrode and which can control whether discharge is generated in accordance with a voltage difference between the first and second electrodes, the method including the step of applying a pulse successively varying from a first voltage to a second voltage, to the first electrode. The step further includes the first step of forming a first region of the pulse in accordance with a first pulse-generation process, and the second step of forming a second region of the pulse in accordance with a second pulse-generation process.
Japanese Patent Application Publication No. 2002-14652 has suggested a method of driving a plasma display panel, in which images are displayed at a certain gray scale by means of a pulse having a increased-width portion, in a certain sub-field.